PROJECT SUMMARY Acute respiratory distress syndrome (ARDS) is a common and devastating complication of critical illness, and effective therapies to prevent ARDS are limited. Vitamin D metabolites have potent immunomodulatory effects and attenuate acute organ injury in animal models. Randomized controlled trials (RCTs) are ongoing to test whether vitamin D3 (vit D3) administration improves outcomes in critically ill patients at risk of ARDS. However, due to the heterogeneous nature of critical illness, patient-specific factors likely play a major role in determining response to vit D3. We will use a precision medicine approach to investigate the clinical, genetic, and biochemical factors that determine response to vit D3 administration in critical illness. This proposal is an ancillary study to the NHLBI-funded Vitamin D to Improve Outcomes by Leveraging Early Treatment (VIOLET) trial, an ongoing, multicenter RCT that is enrolling 3000 critically ill patients at high risk of ARDS and death. Patients are randomly assigned to receive a single enteral dose of 540,000 I.U. of vit D3 or placebo to test whether vit D3 reduces 90-day mortality and acute organ injury. VIOLET is archiving plasma and DNA from all 3000 patients on day 0, and plasma on day 3 from the first 300 patients. We propose to collect plasma on day 3 from an additional 500 patients (Aim 1) and to leverage existing samples (Aims 2 and 3) to test the following hypotheses. In Aim 1 we will use paired plasma samples (n=400) from days 0 and 3 to investigate the clinical factors that affect the pharmacokinetic response to vit D3 administration in critical illness. We will test whether greater severity-of-illness (higher SOFA score), obesity, and acute or chronic liver or kidney disease attenuate increases in plasma 25-hydroxyvitamin D (25D) and 1,25-dihydroxyvitamin D (1,25D) levels from day 0 to 3 in patients who receive vit D3. In Aim 2 we will investigate whether pre-specified common polymorphisms in genes affecting vitamin D metabolism or in vitamin D target genes identify a patient population more likely to benefit from vit D3 administration, assessed by 90-day mortality (primary endpoint) and acute organ injury (secondary endpoint). In Aim 3, we will investigate whether lower plasma levels of fibroblast growth factor-23, an osteocyte-derived hormone that inhibits the conversion of 25D to 1,25D, identify a patient population more likely to benefit from vit D3 administration, assessed by 90-day mortality (primary endpoint) and acute organ injury (secondary endpoint). Development of a precision medicine approach to vit D3 administration in critical illness could have immediate and actionable clinical impact by helping to inform both clinical decision making and the design of future trials of vitamin D metabolites in critical illness.